It has been found that a workpiece moving through a job shop, one in which small orders are scheduled independently of one another, as contrasted with a production shop in which production runs are much longer and more repetitive in nature, spends a small fraction of its time actually being operated on, perhaps 10%. The remainder of the time is spent waiting its turn for the next operation. The benefits of improvement in inventory costs and shop throughput are thought to be potentially very large, but efforts to date have had only limited success in handling the very difficult computational problems associated with scheduling.
Extensive work has been done on this problem, some of which is summarized below:
In the Integer Programming approach, characteristics of routes (like set-ups and lead times) and interactions between routes (like capacity and lot-size) are represented using integer variables. Though integer programs provide powerful models, they require detailed information up front and are computationally intractable for large scheduling problems.
An approach known as Hierarchical Decomposition of Integer Programs is an extension of linear programming which decomposes the problem into two stages: a higher level capacity balancing stage and a lower level route sequencing stage. This approach requires forecasts of capacity requirements up front and usually assumes fixed lead times. This approach is based on infinite capacity.
Simulation techniques allow detailed modeling of the shop under dynamic conditions. They provide a good assessment of release and lot-sizing strategies but fail to regard the schedule with respect to performance measures (for instance, tardiness). With no performance measures it is difficult to assess improvements when making operational decisions.
MRP (Manufacturing Resource Planning) is characterized by infinite capacity backward scheduling. More recently, extensions have provided incremental revisions to the plan based on changes in capacity. But MRP does not alter lead times dynamically based on changes in lot sizes and capacity. This tends to produce highly inflated lead times. An advantage of MRP for large manufacturing enterprises is excellent database support and the corresponding ability to communicate the plan throughout the organization.
Queuing Simulation--Queuing models are good at predicting throughput based on static forecasts of demand. They do not seem appropriate in shops where demand and shop conditions are unpredictable.
OPT, a commercial program available from Synchronous Manufacturing, is a finite scheduling system which focuses on critical shop resources (typically bottlenecks) to constrain capacity. OPT attempts to decrease inventories and increase throughput based on changing lead times. However, OPT cannot be easily extended by its users and thus provides limited `seat of the pants` decision support.